If it was possible I think it would be best to tweak the AI so that kings want to gain control over those counties that are part of their kingdom titles, and to make independent dukes want to conquer lands to claim unheld kingdom titles.
That way if a kingdom fragments, the Kings would try to reconquer or vassalize the lands that make up part of their kingdom. If there was no King (as happens in France at times) then one of the dukes would try to vassalize the others. As it is, the AI never tries to create to rebuild kingdoms as a human player would.
That way if a kingdom fragments, the Kings would try to reconquer or vassalize the lands that make up part of their kingdom. If there was no King (as happens in France at times) then one of the dukes would try to vassalize the others. As it is, the AI never tries to create to rebuild kingdoms as a human player would.